Nitride bearing low-manganese ductile steel



April 13, 1965 HAJIME NAKAMURA NITRIDE BEARING LOW-MANGANESE DUCTILE STEEL 5 Sheets-Sheet 1 Filed April 20, 1962 QII 930C x l hr.,Oil-Cooled-,650C x i5 hrs,Air-Cooled N: 930Cx I hr,Air-Cooled Tempam1ure,C

INVENTOR HAJIME NAKAMURA BY W $3 W ATTORNEY April 13, 1965 HAJIME NAKAMURA 3,173,279

NITRIDE BEARING LOW-MANGANESE DUCTILE STEEL Filed April 20, 1962 5 Sheets-Sheet 2 INVENTOR. HAJIME NAKAMURA ATTORNEY April 13, 1965 Filed April 20, 1962 ASTM Ferrite Grain Size Number HAJIME NAKAMURA 3,178,279

NITRIDE BEARING LOW-MANGANESE DUCTILE STEEL 5 Sheets-Sheet 3 N 930C x I hr., Air-Cooled Q1. 2 930C x lhrpil-cooled;

650Cx l.5 hrs Air-Cooled IO N.

' Metallic Aluminum in Solid Solution,%

INVENTOR. HAJIME NAKAMURA ATTORNEY A ril 13, 1965 Filed April 20, 1962 ASTM Ferrite Grain Size Number HAJlME NAKAMURA 3,178,279

NITRIDE BEARING LOW-MANGANESE DUCTILE STEEL 5 Sheets-Sheet 4 AB! 930Cxl hr,Air-Cooled '2 L L- I0 91 ""E 'A' 4 l I I ,v 1 A 8 BII/ y 6 I I Precipitated Aluminum Nitridefl,

INVENTOR. HAJIME NAKAMURA ATTORNEY April 3, 1965 HUME NAKAMURA 3,178,279

NITRIDE BEARING LOW-MANGANESE DUCTILE STEEL 5 Sheets-Sheet 5 Filed April 20, 1962 lOO X 3% Nital Etch x 1 hr., Oil-Cooled x1 Ductile Steel LMn 1, 930 C 650C ,5 hrs., Air-Cooled ASTM Ferrite rain Size Number, No. 12

PETE- h-OO X Same Specimen as above INVENTOR; 7 /Q 'IL/ne Wafamura,

United States Patent 3,178,279 NITREDE BEARING LOW-MANGANESE DUQTEE STEEL Haiime Nakamura, Tokyo-to, .lapan assignor to Ishiirawm jima-Harirna Jukogyo Kabnshilri Kaisha, Tokyo-to, Japan, a company of Japan Filed Apr. 20, 1962, Ser. No. 189,211 Claims priority, application Japan, May 16, 1%1, 36/ 17 ,37 1 6 Claims. (Cl. 75-124) The low-Mn grade steels are gaining popularity in recent years for use as structural steel. In such application, however, the notch transition temperature must be low, and for that purpose, the ferrite grain size must be uniformly refined. For some time it has been assumed that aluminum oxide is effective for grain refinement and acquiring uniformity, but according to theories precipitated aluminum nitride is more effective in that respect; this theory is in agreement with the discoveries of the present inventor.

Thus it is the object of the present invention to provide low-Mn steels having refined and uniform granular structure, which are best suited for serving as structural steels at low temperatures. Although the low-Mn steel of the present invention exhibits markedly improved ductility due to the presence of appropriate amounts of aluminum nitride as a precipitate, it is in practice not feasible to provide the accurate amounts of aluminum and nitrogen needed for the formation of the desired amounts of aluminum nitride. In every case in practice, either nitrogen or aluminum will be in excess with respect to the other component. When nitrogen is in excess, it will exert a detrimental eifect on the ductility of the steel leaving free nitrogen therein; excess of aluminum, on the other hand, dissolved as solid solution, counteracts the improvement in ductility, too, by coarsening the granular structure. Consequently, if low-Mn steel is to be produced, having a refined and uniform granular structure, the amount of free nitrogen must be limited as much as possible, leaving aluminum in excess over the amount required for the formation of aluminum nitride; however, the amount of metallic aluminum must not exceed a certain limited quantity.

The invention, therefore, has for its object to minimize 3,178,279 Patented Apr. 13, 1965 the detrimental effect of metallic aluminum, by keeping the ratio between aluminum and aluminum nitride at a specified value, and by keeping the free nitrogen content under 0.004% by Weight.

Thus, the present invention relates to low-Mn grade steels containing about 0.05 to 0.25% carbon, less than about 0.60% silicon, about 1.00 to 2.50% manganese, which are characterized by a component consisting of about 0.03 to 0.12% precipitated aluminum nitride, and a component consisting of about 0.003 to 0.15% dissolved metallic aluminium, as well as featuring the granular structure wherein the grain size is over No. 9 in terms of the ASTM ferrite grain size number system. The invention further comprises steels of the composition containing 0.05-0.25 carbon, less than 0.60% silicon, 1.002.5% manganese, 0.030.12% precipitated aluminum nitride, 0.003-0.15% metallic aluminum in solid solution, and 1% of at least one of the elements of the group consisting of nickel, chromium, molybdenum, vanadium and copper, and less than 0.1% boron.

The foregoing percentages are by weight, and the balance of the steel is iron with incidental impurities.

In the following, the full details of the present invention will be disclosed, in which references will be made to figures and photographs, where,

FIGS. 1 and 2 are drawings in which the transition temperature characteristics of various low-Mn grade steels are compared in terms of Charpy V-notch testing method,

FIG. 3 is a drawing to show the relation between the amount of dissolved metallic aluminum in solid solution and the ferrite grain size,

FIG. 4 is a drawing to show the relation between the amount of precipitated aluminum nitride and the ferrite grain size for various low-Mn grade steels with different amounts of dissolved metallic aluminum in solid solution,

FlGS. 5 and 6 are photomicrograms to show the granular structure of a low-Mn grade steel of the present invention.

The chemical composition of low-Mn grade steels representative for those that were submitted to various tests is summarized in Table l. The mechanical properties of said steels are listed in Table 2.

Table 1 Specimen No. C Si Mn P S Ni Cr Mo V LMn 1 0. 09 0.31 1. 35 0. 020 0. 030

Lldn 2 0. 13 0. 49 1. 35 0. 013 0. 007

LMn 3 0. 09 0. 37 1. 17 0. 017 0. 007

LMn 4 r 0. 12 0.31 1. 0. 01 1 0. 009 0. 47 0. 33 0. 12 0. 07

Commercial LMn A. 0. 13 0. 22 1.20 0. 015 0. 010

Commercial LMn B 0. 14 0. 42 1. 12 0. 017 0. 023

Commercial LMn C 0. 12 0. 35 1. 13 0. 016 0. 006 0. 0. 26 0. 12

Specimen N 0. Total Total AlN A1 0 Metallic Free Remarks Al N Al N LMn 1 0. 086 0. 024 0. 067 0.015 0. 034 0.002 Si-Mn type high-tension steel.

LMn 2 0. 113 0.030 0.078 0. 016 0. 053 0. 004 Do.

LMn 3 0.203 0. 023 0. 061 0. 023 0. 151 0. 003 D0.

LMn 4 0. 176 0. 033 0. 085 0. 027 0. 106 0. 003 Si-Mn-Ni-Or- Mo type high-tension steel.

Commercial LMn A 0. 055 O. 009 0. 024 O. 013 0. 032 0. 001 Si-Mn type high-tension steel.

Commercial LMn B 0. 045 0.011 0. 024 0. 005 0. 026 0. 003 D0.

Commercial LMn O- 0. 032 0. 009 0. 021 0. 011 0. 012 0. 002 SiRlVIn-Ni-Crtype high-tension steel.

diflerent amounts of dissolved metallic aluminum in solid solution, the ferrite grain size being reckoned in terms of the ASTM ferrite grain size number system in either case. It will be noted that for the grain size of these steels to be over No. 9, an amount of 0.1% or.-0.23.%. f r. dis:

Table 2 Heat Test piece Guage Yield Tensile Yield Elonga- Reduction Fracture Specimen No. treatment diameter, length, point, strength, ratio, tiou, of area, position mm. mm. kgJmm. kg./mm.- percent percent percent LMnl 40 45.2 52.6 s5.0 33.3 71.9 A 10 40 48.3 58.3 82.8 32.5 78.5 A LM112 N 10 40 43.2 57.0 75.0 31.0 07.5 A 10 40 51.5 03.2 81.5 23.5 73.0 B LMn 3 N 10 40 44.5 53.5 82.8 35.0 70.4 A 10 40 47.8 57.0 22.5 34.3 70.9 A LMn4 N 10 40 54.3 04.9 $3.7 22.5 73.1 A LMnA 14 50 52.4 58.4 89.7 24.0 A LMnB N 14 50 30.7 53.5 68.7 29.0 A LMn o 14 50 52.0 63.9 82.8 22.0 A

*Normalized. tQuench-and-Tempcred.

A brief comment on the nature of steels presented in solved metallic aluminum represents the upper limit in the above tables is now in order. Steels designated as cases of normalization and quench-and-tempering, re- LMn 1 to 4 are the ones in accordance with the present spectively. The amount of precipitated aluminum niinvention. The LMn 1 is a Si-Mn type high-tension 9D tride, on the other hand, must be more than 0.035% or steel containing an amount of precipiated aluminum ni- H 0.0075% in cases of normalization and quench-and-temtride and dissolved metallic aluminum in solid solution pering, respectively, to obtain a refined granular structhat exhibits tensile strengths of the order of 50 and 55 ture of over No. 9 in terms of the above-mentioned grain lag/min. when normalized or quench-and-tempered, resize system, provided that the quantity of dissolved metalspeotively. The LMn 2 has a similar composition to LMn he aluminum is small. It will be further seen that, for 1 except for the carbon content which is slightly higher steels containing a large amount of dissolved metallic than in LMn l, and as a consequence, its tensile strengths aluminum, the necessary limitation imposed upon the conare of the order of 55 and 60 kg/mm. corresponding tent of precipitated aluminum nitride is over 0.10% or to mode of heat treatment of normalization or quench- 0.05% corresponding to the mode or" heat treatment of and-tempering, respectively. LMn 3 is a similar steel. to normalization or quench-and-tempering, respectively. 7 LMn 1 except that its metallic aluminum component is It follows from those empirical findings described considerably higher than in LMn l, and the tensile above, and the fact that the cooling rates applied on specistrengths are almost equal to those of the latter. LMn 4 mens involved in the various tests were rather faster than is again another Si-Mn type steel except modified by what would be realized in practice, a use of optimum small amounts of Ni, Cr, Mo and V each to improve the compositions ranging from about 0.03 to 0.12% for pretensile strengths which are of the order of 60 or '70 rripitated aluminum nitride, and from about 0.003 to kg./rnm. when normalized or quench-and-tempered, re- 0.15% for dissolved metallic aluminum in solid solution, spectively. is proper for successrul production of fine-grained, low- Steels designated as LMn A, LMn B and LMn C are temperature ductile low-Mn grade steels. so-called low-Mn grade steels that were acquired from 4Q; In FIG. 2, the transition temperature characteristics of commercial market. The LMn A is equivalent to LMn two different low-Mn grade steels, as normalized, one due 2, LMn B to LMn l, and LMn C to LMn 4, respectively. to the present invention, the LMn 4, and the other a com- The transition temperature curves obtained from lowmercial steel, the LMn C, are compared. It is to be seen Mn grade stels of the present invention and those from that the steel of the present invention is far more ductile commercial steels of the same grade are compared in than its commercial counterpart as evidenced by the rela- FIG. 1. It will be seen from said figure that the low-Mn tive position or" the transition temperature curves, the one grade steels containing precipitated aluminum nitride of representing the steel to in accordance with the present more than 0.03% all have transition temperature curves invention lying at far lower side in the temperature scale that are in a temperature region that is lower by about and far above along the impact value scale with respect 40 to 80 C. as compared with those of commercial to the commercial steel. counterparts, and, moreover, the absorbed energy at frac- FIGS. 5 and 6 are photomicrograms showing the microture in the former varieties is far larger than that in the structure of a low-Mn grade steel of the present invention, latter at any temperature tested. the LMn 1. It is to be noted the grain size thereof is It will be noted in FIG. 1, furthermore, that the transiabout No. 12 in terms of the ASTM ferrite grain size tion temperature curves of LMn 3 is shifted towards the system, higher t m t side with f to flfose of LMn The low-Mn grade steels due to the present invention The i shlft of the curve and about are all heat-treatable in that the granular structure thereof for h .3. of normahzailo} and quenchand can be refined further, hence the low-temperature ductility pf i l'jcspecfively- Thus brltflgness of M 3 more improved, as compared with the same stock but in i evident w th regard to LMn l in either state of h al a state of as-rolled or as-forged, by executing a heat treatg i- 51mg no malor (imagine in chemical ment process consisting of rapidly cooling said steel from PQBitlOll 1S detefited except the amount of a temperature Within a temperature range of maximum sowed metalhc alummum between two Steels as precipitation of aluminum nitride, tobe followed, if dey be j d from Table a cn1us1nthatuch sired, by reheating it to a lower temperature for brittlement as mentioned above should be attributed to tempering the excessive amount of dissolved metallic aluminum in The tamperature of maximum Precipitation of alumi LMn? 1s Propernum nitride is about 800 C. to 1050 C., whereafter g gi ii g g g gi izgifiigfi g ggg i t i fi gii f V the steel is rapidly cooled in a fluid medium to retain the While FIG. 4 Shows a relation between ferrite grain state of preclpitationh to tlge fullest extent. if desired, size and amount of precipitated aluminum nitride for me Steel may be furt er Su jected to a tempering Stgp at a temperature between about C. and 750 C.

I claim:

1. A nitride bearing low manganese ductile steei consisting essentially of, by weight, 0.05% to 0.15% carbon, less than.0.60% silicon, 1.0% to 2.5% manganese, 0.03%

to 0.12% aluminum nitride, 0.003% to 0.15% metallic aluminum in solid solution, balance iron with incidental impurities, said steel havng a grain size liner than number 9 in terms of the ASTM ferrite grain size numbering system.

2. A nitride bearing low manganese ductile steel consisting essentially of, by weight, 0.05% to 0.15% carbon, less than 0.60% silicon, 1.0% to 2.5% manganese, 0.03% to 0.12% precipitated aluminum nitride, 0.003% to 0.15% metallic aluminum in solid solution, less than 1% of at least one element selected from the group consisting of nickel, chromium, molybdenum, vanadium, and copper, less than 0.1% boron, balance iron with incidental impurities said steel having a grain size finer than number 9 in terms of the ASTM ferrite grain size numbering system.

3. Method for the heat treatment of steel having a chemical composition according to claim 1, comprising the steps of heating said steel to a temprature between about 800 and 1050" C. for maximum precipitation of aluminum nitride, and then cooling the steel rapidly in a fluid medium to retain the state of precipitation of the nitride to the fullest extent.

4. Method for the heat treatment of steel having a chemical composition according to claim 2, comprising the steps of heating said steel to a temperature between about 800 and 1050 C. for maximum precipitation of aluminum nitride, and then cooling the steel rapidly in References Cited by the Examiner UNITED STATES PATENTS 2,679,454 5/54 Ofienhauer -128.5

FOREIGN PATENTS 808,556 2/59 Great Britain.

OTHER REFERENCES Geil et al.: Journal of Research of the National Bureau of Standards, vol. 48, N0. 3, March 1952, pages 193-200.

Case et al.: Al In Iron and Steel, 1953, pages 79-89 and -132; published by John Wiley and Sons, Inc., New York, N.Y.

DAVID L. RECK, Primary Examiner.

MARCUS U. LY ONS, ROGER L. CAMPBELL,

HYLAND BIZOT, Examiners. 

1. A NITRIDE BEARING LOW MANGANESE DUCTILE STEEL CONSISTING ESSENTIALLY OF, BY WEIGHT, 0.05% TO 0.15% CARBON, LESS THAN 0.60% SILICON, 1.0% TO 2.5% MANGANESE, 0.03% TO 0.12% ALUMINUM NITRIDE, 0.003% TO 0.15% METALLIC ALUMINUM IN SOLID SOLUTION, BALANCE IRON WITH INCIDENTAL IMPURITIES, SAID STEEL HAVING A GRAIN SIZE FINER THAN NUMBER 9 IN TERMS OF THE ASTM FERRITE GRAIN SIZE NUMBERING SYSTEM. 